Electrical resistivity and length changes during precipitation in Fe–V–C alloy

Abstract

Electrical resistivity measurements at 77 K have been used to follow vanadium carbide precipitation in an Fe–0·12C–0·02Mn–0·46V alloy during isothermal aging between 598 and 800°C after solution treatment at 1200°C. The contributions to the resistivity from the lattice, grain boundaries, dislocations, carbon, and vanadium were evaluated and found to agree well with the measured values (36 nΩm at short aging times, 14–16 nΩm at long times). To evaluate the effect of cementite precipitation at lower temperatures, specimens were tempered at 350°C after the isothermal aging heat treatment. After correcting for the effect of the γ → α phase transformation, volume fractions of vanadium carbide were obtained assuming a linear relationship with resistivity. 80–86% of the vanadium was estimated to precipitate in the two-phase region between 598 and 800°C. A comparison of resistivity and dilatometry was made by measuring length changes between 652 and 792°C. Above 730°C the results showed good agreement, but below this temperature the length changes due to the formation of cementite masked the effect of vanadium carbide precipitation.